The present invention relates to an engine idle control valve for controlling an amount of auxiliary air flow to be supplied to an engine from a bypass passage which bypasses an intake throttle valve during an idle condition of the engine, and more particularly to a fail-safe for the engine idle control valve.
A conventional engine idle control valve of such type suffers from the following problem. In the case where a drive signal is not supplied due to the fact that, for example, a signal line through which the drive signal is supplied is broken down, the idle control valve is held under a fully closed condition so that almost no auxiliary air is supplied through a bypass passage to thereby reduce an engine RPM (revolutions per minute) and to lead to an engine stall.
Conventionally, there has been disclosed an engine idle control valve in which, in order to overcome the above-noted problem, even if the drive signal is supplied to the engine idle control valve, as shown in FIG. 8, such an amount Q1 of auxiliary air flow in order that any engine stall is not caused is supplied through the bypass passage (see, for example, Japanese Patent Unexamined Publication 59-150939 (i.e. U.S. Pat. No. 4,494,517, DE-)S 3, 23 44 68)).
However, in such an engine idle control valve, in the case where the signal line through which the drive signal is supplied thereto would be short-circuitted with, for example, a battery voltage line, a battery voltage (held at about 14V), i.e., an maximum drive signal is always supplied to the control valve. Accordingly, the engine idle control valve is held under the fully opened condition. As a result, the amount of auxiliary air flow to be supplied through the bypass passage is kept at the maximum amount Q.sub.max of auxiliary air flow. Accordingly, the engine RPM is abnormally increased.